This invention relates in general to feedback control systems, and more particularly to a method and apparatus to control the operation of equipment acting on a moving web in coordination with the location of printed material on the web.
In general, it is well known to use feedback loops to control an operation in response to one or more sensed inputs. In the printing industry it is necessary to coordinate the position of a rapidly moving web of paper with the operation of printing cylinders. A common approach is to print a series of registration marks along the web, and then scan the marks. A basic approach introduced by Hurletron is to scan the web for brief intervals in an inspection zone centered on the anticipated position of the mark. A lead inspection zone precedes the mark, followed by a dead zone and a lag inspection zone. Sensed misregistrations in the dead zone are ignored to avoid hunting. Also, the sensed mark is validated; the same mark must be seen in the inspection zone for a predetermined number of successive scans. This control arrangement works well in a printing press, but it works well because the position of a mark does not move relative to the controlled cylinder. It is synchronous with the cylinder. Where the operation is asynchronous, as where there is a cumulative error or an out-of-register splice, the control system does not see the same mark the required number of successive times. No mark is validated, and the system loses control.
In off-press web finishing, e.g. in folding, perforating, gluing, and cutting a pre-printed web, the location of the marks is often not well-determined; they are not synchronous with the cylinder. As a result, conventional on-press controls do not function well for off-press applications, or more generally, for applications where the position of the mark can change significantly, whether due to localized variations or to cumulative variations in the length of the impressions printed on the web. These variations occur because once the web is released from the tight control of the press, variations in web tension due to printing, heating, chilling, handling and atmospherics cause corresponding variations in the dimensions of the web, and hence of the impressions printed on the web. These variations are particularly acute when the printed web is rewound, stored, and then unwound at a later time to be run through a finishing line. They tend to make impressions run consistently longer or shorter than their initial printed length.
U.S. Pat. Nos. 5,129,568 and 5,224,640 to Fokos et al. disclose an off-line web finishing system which overcomes many of the problems of prior art systems that maintain registration by stretching the web, introducing variations in the path length, phasing the operation of the function cylinders, or some combination of these approaches. These prior art systems and their deficiencies are detailed in these patents. With cumulative (synchronization) error, the phasing gears usually are not able to keep up with the error since the error may be greater than the maximum correction rate possible without introducing over correction or hunting. With path length changes, compensating rolls or equivalent structures soon reach their operational limits and cannot accommodate further repeated errors of the same type. Moreover, the dynamic response of known systems often results in system instabilities such as hunting.
The Fokos et al. patents solve these problems in the way machinery in the line is driven. In one form, these patents teach using 1) two drive shafts, a main shaft for web transport and a secondary shaft for the function cylinders, and 2) a continuous ratio adjustment between the speed of operation of the lineshaft to correct for cumulative error. One line drives the other line through a variable transmission or the like to produce the continuous ratio adjustment. Conventional phasing gears at each function cylinder can provide additional adjustment to deal with localized errors. These patents also disclose independent drive motors at each function cylinder, also operated with continuous ratio adjustments with respect to the web transport. While these drive systems provide significant performance advantages over the prior art, they nevertheless have certain drawbacks.
First, transmissions, lineshafts, and other equipment connected between the shafts and driven cylinders introduce some degree of play, which is in itself a source of misregistration. Second, the dynamic response of these mechanical systems is limited in part by the mass of the components and any play or resilience in the system. Third, there is additional cost for a second lineshaft and its installation. Fourth, these systems with known electronic controls do not respond well to massive errors, as occur in connection with splices. The entire line adjusts as soon as a splice is detected at the head of the line. Good impressions downstream are processed out of registration, and continue to be processed out of register, while the system regains synchronous operation. Fifth, the system does not accommodate well to large changes in the line speed, e.g. operation at 500 and 1,000 feet per minute (fpm). The problem is even worse if the speed ranges from manual mode set-up speeds of under 100 fpm to normal line operating speed of 1,000 to 2,000 fpm in an automatic mode. Scanning rates and dynamic system responses that produce satisfactory results at one speed do not function as well at a much different speed. The flexibility of known systems is thus constrained. Sixth, the angular position of the function cylinders in which they act on the web must be initialized into a synchronous start with the registration marks. Seventh, after the initial synchronization the operator has a display of the instantaneous registration error, but there is no indication of the history of the correction process, e.g. its momentum. Nor does any known display assist the operator in finding a registration mark if it is lost, as due to a splice.
In control systems, it is known to use proportional (P) controls, that is, controls where the degree of correction is varied in proportion with the magnitude of a sensed error. A large sensed error produces a larger correction than a small sensed error to hasten the return of the system to a desired condition, e.g. in register. Integral (I) control is also known, where the correction responds to the measured average or integrated error over some preselected prior period of operation. Derivative control is also well known and often used in combination with PI control to form a class of control, PID. The derivative control senses and responds to the rate at which a correction occurs.
As applied to web registration control where there is cumulative error, none of these forms of control have been successful heretofore. Proportional control with cumulative error occurring at high speeds requires large proportional gains. A high gain is needed to deal with the comparatively large recurrent errors and to achieve the necessary register accuracy. But the high gain causes the correction to overshoot the set point (where the system is in register). Hunting occurs as the system oscillates about the set point, or while the system looks for a registration mark which has been over-corrected to a degree that it falls out of the preset inspection zone. Integral controls, in turn, when responding to cumulative errors of the same type, e.g. impressions that are consistently printed long or short, tend to develop a momentum that causes the error correction to overshoot a set point and then oscillate about it, or lose it entirely. The system may settle on the set point, but only after a fairly lengthy interval during which time thousands of feet of printed web are processed out of register and must be scrapped. PID controls have not solved these fundamental problems since they do not automatically accommodate for this change in the error sampling rate that is inherent in register control systems.
U.S. Pat. No. 4,994,975 to Minschart describes a system for off-line web finishing that automatically initializes synchronization between register marks and a processing machine. It selects a registration mark and stores in memory a digital sequence that describes at least one characteristic of the mark. Subsequently detected signals continue to be analyzed and stored in the same manner. Periodically the stored contents of memory are analyzed to locate the selected mark. Finally, the deviation of this selected mark is calculated from 1) a set point position and 2) the position of an operating element of an associated one of the processing machines.
While Minschart recites the use of proportional (P), proportional-integral (PI) and proportional-integral-differential (PID) controllers to produce error correction signals, all three forms are described as suitable and of well-known design. The control advantages purportedly derive from the computer analysis of memory-stored marks to one another and to the position of the machine. This control arrangement does not address or solve the hunting, and validation problems noted above with respect to conventional P or PI systems. The system also requires repeated pattern recognition, e.g. an analysis of a mark by its width, direction of travel and shape. This analysis requires substantial signal processing and capability and is sensitive to irregularities in the printing of the marks or other printed indicia. Moreover, the accuracy of the control remains limited by the ability of an encoder to determine at any instant the precise angular position of the machine element carrying out the process.
It is therefore a principal object of this invention to provide a control system (method and apparatus), particularly one for off-line web finishing, that corrects for both localized and cumulative errors, and is highly stable even with substantial changes in line speed such as variations of a factor of twenty or more.
Another object is to provide a control system which has the foregoing advantages and which can operate with an asynchronous start and which can quickly and reliably accommodate splices, cumulative error, or other asynchronous operating conditions, during operation.
A further object is to provide a system that is highly flexible to accommodate i) different types of web finishing systems such as single or dual shaft drives, variable ratio or fixed ratio systems and (ii) the control of different processes such as registration, cutoff length control, and infeed tension control, and (iii) different modes of control, e.g. manual or automatic.
Yet another object is to avoid the need for a dead zone around the set point.
Still another object is to provide a control system with all of the foregoing advantages that significantly reduces capital cost for hardware (e.g. position transducers, in-register splicers, dual lineshafts) as well as hardware installation time and set up time.
Yet another object of the invention is to provide a system with a graphic display that provides a convenient, real-time, analog display of the correction process.
A further object is to provide an arrangement for greatly enhancing the accuracy of angular position information from an encoder or other standard angular position transducer.